1. Field of the Invention
The invention relates generally to the field of marine seismic surveying. More specifically, the invention relates to seismic energy sources for marine surveying that have enhanced low frequency energy output.
2. Background Art
Marine seismic surveying includes deploying a seismic energy source in a body of water, actuating the source and detecting seismic energy reflected from acoustic impedance boundaries in rock formations below the water bottom. A typical seismic energy source used in marine seismic surveying is an air gun array. Air gun arrays include a plurality of individual air guns, each of which is connected to a source of compressed air or gas. Each air gun includes a chamber to store the compressed air or gas, at a selected pressure, and a remotely operable valve to release the compressed air or gas at predetermined times. The frequency of the seismic energy generated in the water by release of the compressed air or gas is related to, among other parameters, the volume of the chamber. An example air gun is described in U.S. Pat. No. 4,472,794 issued to Chelminski.
It is known in the art that the lowest frequency of an air gun array is the reciprocal of the bubble oscillation period of the largest air gun. The modified Rayleigh-Willis formula accurately predicts the bubble oscillation period for a single air gun as follows:
                    T        =                  K          ⁢                                                    P                                  1                  /                  3                                            ⁢                              V                                  1                  /                  3                                                                                    (                                                      P                    0                                    +                                      ρ                    ⁢                                                                                  ⁢                    gD                                                  )                                            5                /                6                                                                        (        1        )            
where T is the bubble period, P is the air or gas charge pressure of the air gun, V is the volume of the air gun chamber, P0 is atmospheric pressure, ρ is the density of water, g is Earth's gravitational acceleration, D is the depth of the air gun in the water, and K is a constant that depends on the units of measure of all the foregoing parameters.
For a particular depth D and a particular air gun chamber pressure P, the bandwidth of an air gun array may be extended at the low frequency end by increasing the chamber volume of the largest air gun. From equation (1) it is apparent that the bubble period is proportional to the cube-root of the chamber volume of the largest air gun in the array. As a practical matter, air gun arrays known in the art do not produce substantial seismic energy below about 8 Hz. While it is possible to produce air gun arrays that would produce substantial seismic energy below 8 Hz, the chamber volume of air guns capable of producing such low frequency seismic energy would be so large as to require substantial increase in the size of the source of compressed air or gas, among other practical limitations. The bubble period equation shows that it is equally possible to increase the bubble period using the same chamber volume by increasing the charge pressure. The issue with this possibility is that airguns become much more unreliable at higher charge operating pressures. Accordingly, there is a need for marine seismic energy sources that emit more energy at frequencies below about 8 Hz that would not require increased volumes or pressures of compressed air or gas.